diff --git a/2015/13/dune b/2015/13/dune
index 3774870..c9ad225 100644
--- a/2015/13/dune
+++ b/2015/13/dune
@@ -1,6 +1,6 @@
 (library
  (name year2015_day_13_solution)
- (libraries angstrom async bignum core core_kernel.composition_infix
+ (libraries angstrom async bignum core core_kernel.composition_infix euler
    expect_test_helpers_async re topological_sort yojson)
  (inline_tests
   (deps aoc.in))
diff --git a/2015/13/main.ml b/2015/13/main.ml
index e795ba0..84ca965 100644
--- a/2015/13/main.ml
+++ b/2015/13/main.ml
@@ -2,34 +2,6 @@ open! Core
 open! Async
 open! Import
 
-(* FIXME: copy *)
-let next_permutation array ~compare =
-  let rec find_first_sorted_from_end pos =
-    if pos < 0
-    then None
-    else if compare array.(pos) array.(pos + 1) < 0
-    then Some pos
-    else find_first_sorted_from_end (pos - 1)
-  in
-  match find_first_sorted_from_end (Array.length array - 2) with
-  | None -> false
-  | Some to_increase ->
-    let rec find_next_largest pos current_min arg =
-      if pos >= Array.length array
-      then arg
-      else if compare array.(pos) current_min < 0
-              && compare array.(pos) array.(to_increase) > 0
-      then find_next_largest (pos + 1) array.(pos) pos
-      else find_next_largest (pos + 1) current_min arg
-    in
-    let swap_with =
-      find_next_largest (to_increase + 1) array.(to_increase + 1) (to_increase + 1)
-    in
-    Array.swap array to_increase swap_with;
-    Array.sort array ~pos:(to_increase + 1) ~compare;
-    true
-;;
-
 let parse_line =
   let parser =
     let open Angstrom in
@@ -78,7 +50,7 @@ let find_highest_gain table ~self =
   let max_gain = ref Int.min_value in
   let rec loop () =
     max_gain := Int.max !max_gain (count_gain set table ~self);
-    if next_permutation set ~compare:String.compare then loop ()
+    if Euler.Sequences.next_permutation_inplace set ~compare:String.compare then loop ()
   in
   loop ();
   !max_gain